1. Field of the Invention
This invention relates to a method of auto-focussing and a system therefor, and more particularly to a method of auto-focussing and a system therefor, wherein a high frequency component is extracted from image signals obtained from an image sensing element such as a charge coupled device (CCD) as a focus voltage and focussing is performed automatically in such a manner that the focus voltage attains the maximum level.
2. Description of the Related Art
There has heretofore been such an auto-focussing system wherein, based on the fact that the voltage level of a high frequency component of the image signals obtained by. photographing an object corresponds to the degree of precision of a picture image, the high frequency component from the image signals is extracted as a focus voltage and the focus lens is driven in such a manner that this focus voltage attains the maximum level, whereby the position of the lens is adjusted to the focussing position.
The auto-focussing system of this kind includes a type first, in which the focus voltages at two points are successively compared in level, whereby the presence of a shift in focussing and a moving direction of the lens are detected on the basis of the magnitudes of the focus voltages, so that the lens is moved, and a second type, in which a distance to a focussing position is calculated from a differential value of the focus voltage at an optional position and the lens is driven on the basis of the thus calculated value. The former is known as a mountain climbing servo type and described in detail in the NHK Technical Research Report 1965, volume 17, No. 1, the consecutive number of volumes No. 86, page 24, or the Television Society Technical Report ED No. 675, page 7 and so forth. Furthermore, the latter is described in Patent Application Laid-open No. 62-272217 and Patent Application Laid-Open No. 62-2722218 and so forth.
Now, in the case of the former i.e., the mountain climbing servo type, the inclinations of the focus voltages are successively detected, whereby the peak of the focus voltages (focussing position) is sought while the lens is being moved, thus presenting the disadvantage of taking a lot time before the focussing position is reached.
On the other hand, in the case of the latter, the differential is performed once or twice, whereby the width of differential affects the accuracy of calculation. The smaller the width of differential is, the better the accuracy of the calculation becomes, however, the smaller the width of differential is, the smaller the change in the focus voltage becomes, so that the detecting accuracy of the focus voltage is affected.
Furthermore, in the case of the latter, the high frequency component from the image signals is extracted as the focus voltage and the distance to the focussing position is calculated on the basis of this focus voltage. However, the result of the calculation is affected by the actual state of the object. Accordingly, depending on the result of the calculation based on the focus voltage obtained from one frequency component, there are cases where both the movements of the lens and the calculations of the distance to the focussing position after the movements of the lens must be repeated a multitude of times until the distance to the focussing position can enter the allowable circle of confusion.